WASHINGTON (CNN) --With 13 confirmed attacks, 10 of them fatal, the sniper shootings that have terrorized the Washington area for three weeks have spawned the largest criminal investigation the region has ever seen.

A conservative estimate would put at 1,000 the number of officers and experts from various federal, state and local law enforcement officers assigned to the case, and the size of the investigation grows with each new development -- and shooting -- in the case.

"We've been involved in a wide variety of investigations, but nothing that has reached this scope," said Lucille Baur, a spokeswoman for the Montgomery County, Maryland, Police Department, the lead agency in the sniper probe. "This is really unprecedented."

While the nation's ongoing terrorism probe in the wake of the September 11, 2001, attacks against the United States is obviously larger, the sniper investigation stands out in the region because of the number of law enforcement jurisdictions participating.

Baur said she could not offer an exact count of the people working on the investigation from all jurisdictions, because the number changes with each new development. Federal agencies alone account for at least 800 people.

For example, the sniper shot a man Saturday night in Ashland, Virginia -- about 90 miles south of Washington -- drawing authorities in Hanover County, Virginia, into the case.

The scope of the investigation was underscored again Tuesday with the shooting death of a Montgomery County bus driver.

Authorities are treating the slaying as if it were the latest attack from the sniper, although ballistics tests to confirm -- or rule out -- that connection are not yet completed.

At a news conference to discuss Tuesday's killing, Montgomery County Police Chief Charles Moose, the head of the task force investigating the sniper attacks, appeared with officials from the FBI and the Bureau of Alcohol, Tobacco and Firearms to talk about the case.

"We remain concerned about the safety of all the people in our region," said Moose, saying the killer has shown no regard for age, race, gender, profession or location.

Police in Montgomery County, where the shootings began October 2, are working with law enforcement offices in Maryland, Virginia and the District of Columbia, as well as several federal agencies.

Even the Defense Department is assisting in the case, allowing Army airborne surveillance and reconnaissance aircraft to be used in the hunt for the sniper.

While the question of whether the investigation ought to be federalized or moved to a central location -- namely Washington -- has been raised several times, authorities said Tuesday there was no need for such a move.

"This continues to be a joint investigation by a large number of state, local and federal law enforcement agencies," FBI agent Gary Bald said, describing the level of cooperation among the various offices as "100 percent."

Moose has become the most visible face of the investigation. Bald and Mike Bouchard of the ATF usually appear with Moose at his news conferences.

Other jurisdictions participating in the investigation include police departments in the cities of Gaithersburg and Rockville -- both in Montgomery County -- and police in Maryland's Prince George's County and Virginia's Fairfax, Prince William, Spotsylvania and Hanover counties. The Maryland-National Capital Park Police are also involved in the investigation.

Federal personnel committed to the case include approximately 400 FBI agents, 250 agents from the ATF, 50 Secret Service agents and about 100 deputy marshals from the U.S. Marshals Service, authorities said.

Officials at the U.S. Park Police said they too are providing assistance, but did not have specific numbers. The U.S. Customs Service has deployed helicopters to participate in surveillance flights in the area.

"It's a situation where each of the agencies brings their particular areas of expertise to bear on this case," said the FBI's Bald.

"I can tell you that the cooperation that we have is unprecedented in this case, and we feel very comfortable that the platform that we've developed, and the cooperation that we've received from the other jurisdictions, is exactly what we need to be able to pursue this case successfully."